(Just increase the ratio of yoghurt to milk to make a larger batch)
11. Add milk to slow cooker
22. Cover and heat on Medium for approximately 2 hours, or until milk reaches 180F / 82C on a candy thermometer.
33. Remove lid and remove bowl from slow cooker, and allow to cool until it reaches 110F / 43C. How long this takes depends on ambient temperature, check in after 1 hour. If it drops too far, gently heat up again until just 110 / 43.
44. Take out a small scoop of the warm milk and place in a bowl. Whisk in the yoghurt.
55. Pour the mix back into the bowl and gently whisk into yoghurt to mix through.
66. Cover and keep in a warm place for at least 6-8 hours. The aim is to keep the bowl as close to 110F / 43C as possible during that time, so the yoghurt can incubate. Try wrapping the slow cooker in a towel and placing in a warm place, or placing in an oven with only the light on (no other heat).
77. It is done once it has thickened (may happen after 6-8 hours, depending on how warm it is, but is ok to incubate for up to 24 hours).
88. To make thicker (greek-style) yoghurt, strain the mix gently through a clean tea towel to remove the excess whey.
10I was inspired by this post and you can find lots of tips on success yoghurt-making here: http://girlsguidetobutter.com/2010/02/crock-pot-yogurt/